Process for reducing monomer content in N-vinyl compound polymers

ABSTRACT

A method for residual monomer diminution by which a residual monomer is speedily removed from an N-vinyl compound polymer or the like without posing a problem such as an increase in ash content; and a process for producing an N-vinyl compound polymer solution or powder which has a regulated pH and is free from a decrease in pH with time. 
     The method comprises adding an organic acid having a boiling point of 140° C. or higher at ordinary pressure to anaqueous solution of an N-vinyl compound polymer. The process comprises adding an organic base to an aqueous N-vinyl compound polymer solution having a pH lower than 7.0 to thereby neutralize the solution and regulate the pH thereof. Those operations are conducted in a reaction vessel in which a gaseous phase is regulated so as to have an oxygen concentration of 5.0% by volume or lower.

This is a divisional of application Ser. No. 09/920,745 filed Aug. 3,2001 now U.S. Pat. No. 6,617,420; the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for diminishing residualmonomers contained in an N-vinyl compound polymer. The invention furtherrelates to a process for producing an N-vinyl compound polymer solutionor powder having a regulated pH.

DESCRIPTION OF THE RELATED ART

N-vinylpyrrolidone, N-vinylcaprolactam, N-vinyloxazolidone,N-vinylacetamide, N-vinylformamide, N-vinylimidazole and the like haveadvantages of biocompatibility, safety, hydrophilicity, etc., andtherefore are utilized in various uses. In particular,N-vinylpyrrolidone makes it possible to synthesize polymers having abroad range of molecular weight of about 10 to 100 in terms of K valueas determined by the Fikentscher's method, and such polymers areextensively used in various fields such as medicines, cosmetics,pressure-sensitive and other adhesives, coatings, dispersants, inks andelectronic parts. Further, crosslinked vinylpyrrolidone polymers areuseful as a water-absorbing resin in various applications where waterabsorption and water retention are required, e.g., paper diapers. By theway, there is the case that unpolymerized monomers may often remain inthose N-vinyl compound polymer products. Amount of the monomer remainedis generally in a level of 1% to several hundreds ppm, but even such asmall amount of the residual monomers poses problems concerning, inparticular, a fear of toxicity and odor. In recent years, there is agrowing desire for diminishing the residual monomers contained inN-vinyl compound polymer products for use especially in medicines andcosmetics. Methods for removing a residual monomer from avinylpyrrolidone polymer have been proposed. For example, JP-B-7-59606(the term “JP-B” as used herein means an “examined Japanese patentpublication”) proposes a method in which an aqueous vinylpyrrolidonepolymer solution is treated with an adsorbent. Further, JP-W-7-503749(the term “JP-W” as used herein means an “unexamined published PCTapplication”) proposes a method which comprises adding carbonic acid,formic acid, acetic acid, phosphoric acid or sulfuric acid to an aqueousvinyllactam polymer solution and heating the resulting mixture at 50 to150° C. Use of an inorganic hydroxide as an example of a neutralizingagent to be used after the acid treatment is described therein.

The method disclosed in JP-B-7-59606 has a drawback that the aqueoussolution to be treated by this method should be regulated so as to havea viscosity of 200 mPa.s or lower and, hence, the method is unsuitablefor use in the treatment of polymers having a high viscosity. Further,for use of an adsorbent such as ion-exchanged resins, complicatedprocedures such as pre-washing or regeneration treatment of adsorbentmust be conducted, resulting in increase of production costs.

The method disclosed in JP-W-7-503749 has a drawback that when carbonicacid, formic acid or acetic acid, which each are a volatile acid, isused at a reaction temperature of 80° C. or higher, then the acidvolatilizes to escape from the system or move to the gaseous phase. As aresult, it becomes impossible to maintain the pH of the solution at adesired value, making it difficult to speedily diminish the residualmonomer. On the other hand, when the reaction temperature is reduced to80° C. or lower, it becomes difficult to speedily diminish the residualmonomer although acid volatilization is reduced. Furthermore, there havebeen cases where the acid volatilizes during the drying of the polymersolution, making it impossible to obtain a dried polymer having adesired pH value. A further problem of the proposed method is that useof an inorganic acid such as phosphoric acid or sulfuric acid results inan increased ash content in the resulting aqueous solution or in thedried solid. It has further been found the problem that where oxygen hasentered in a reaction system, molecular weight of polymer reduces.

Moreover, there is a problem that in the case where an acid is added toan N-vinyl compound polymer to lower than pH of the system to below 7.0,the polymer deteriorates with the lapse of time, resulting in a decreasein molecular weight and, in the case of an aqueous solution, a decreasein solution viscosity. In addition, there has been a further problemthat when an aqueous polymer solution having a pH lower than 7.0 isdried at a high temperature exceeding 100° C., the molecular weight ofthe polymer decreases. JP-W-7-503749 proposes a technique in which thepolymer treated with an acid is neutralized with an inorganic hydroxide.However, use of an inorganic hydroxide poses a problem that theresultant aqueous solution or dried solid has an increased ash content.Furthermore, the present inventors have found that there is a problemthat the presence of an inorganic hydroxide lowers the pH of the aqueouspolymer solution or of the dried solid during drying or with the lapseof time and even deteriorates the polymer.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a method fordiminishing residual monomers contained in N-vinyl compound polymerswhich can speedily diminish the amount of residual monomers to less than100 ppm, preferably less than 10 ppm, and also is free from thedrawbacks described above.

Another object of the invention is to provide a process for producing anN-vinyl compound polymer solution or powder having a regulated pH andfree from the drawbacks described above.

As a result of intensive investigations to overcome the above drawbacks,it has been found that a method effective for diminishing a residualmonomer comprises adding an organic acid having a boiling point of 140°C. or higher at ordinary pressure to an aqueous solution of an N-vinylcompound polymer. The organic acid having a boiling point of 140° C. orhigher at ordinary pressure according to the invention does notvolatilize even at high temperatures and functions to maintain the pH ofthe solution constant. Consequently, the residual monomer can bespeedily diminished, whereby a polymer solution or a dried polymer eachhaving a desired pH can be easily obtained. Furthermore, an ash-freeaqueous polymer solution and an ash-free dried solid can be obtained.

A method for diminishing a residual N-vinyl compound monomer has furtherbeen found, which comprises adding at least one acid to an aqueoussolution of an N-vinyl compound polymer, wherein the gaseous phase inthe reaction vessel is regulated so as to have an oxygen concentrationof 5.0% by volume or lower. When this method of the invention in whichthe gaseous phase in the reaction vessel is regulated so as to have anoxygen concentration of 5.0% by volume or lower is used, for example,vinyllactam polymer is prevented from suffering a decrease in K value(determined by the Fikentscher's method) even when treated with an acidat high temperatures. Thus, a vinyllactam polymer having a desired valueof K can be obtained with satisfactory reproductivity.

As a result of further intensive investigations, there has been found aprocess for producing an aqueous N-vinyl compound polymer solution whichcomprises adding an organic base to an aqueous N-vinyl compound polymersolution containing an acid and having a pH lower than 7.0 to therebyneutralize the solution and regulate the pH thereof. By using an organicbase according to this process of the invention, not only an ash-freeaqueous solution and an ash-free dried solid can be obtained, but alsothe pH of the pH-regulated aqueous solution can be maintained constantover a prolonged period of time. Thus, an N-vinyl compound polymer whichsuffers no change in K value or pH during drying and during storageafter drying and has excellent stability with the passage of time can beobtained.

The invention provides a method for diminishing a residual monomer whichcomprises adding an organic acid having a boiling point of 140° C. orhigher at ordinary pressure to an aqueous solution of an N-vinylcompound polymer.

The invention further provides a method for diminishing a residualmonomer which comprises adding at least one acid to an aqueous solutionof an N-vinyl compound polymer, wherein a gaseous phase in the reactionvessel is regulated so as to have an oxygen concentration of 5.0% byvolume (hereinafter referred to as “vol %”) or lower.

The invention furthermore provides a process for producing an aqueousN-vinyl compound polymer solution which comprises adding an organic baseto an aqueous N-vinyl compound polymer solution having a pH lower than7.0 to thereby regulate the pH of the solution.

The invention still further provides a process for producing an N-vinylcompound polymer powder which comprises drying the aqueous polymersolution which has undergone the pH regulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in detail below.

Examples of the N-vinyl compound polymer include homopolymers ofN-vinylcaprolactams such as N-vinyl-2-pyrrolidone,N-vinyl-5-methyl-2-pyrrolidone, N-vinyl-2-piperidone,N-vinyl-6-methyl-2-piperidone, N-vinyl-ε-caprolactam andN-vinyl-7-methyl-ε-caprolactam; N-vinylamides such as N-vinylacetamide,N-vinylformamide, N-vinyl-N-methylacetamide andN-vinyl-N-methylformamide; N-vinyl imides such as N-vinylmaleimide andN-vinylphthalimide; and N-vinyl compounds such as N-vinyloxazolidone;and copolymers.

The copolymers are constituted of the above N-vinyl compounds andcomonomers copolymerizable therewith. The comonomers to be copolymerizedare not particularly limited. Examples thereof include 1) (meth) acrylicesters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate, cyclohexyl (meth)acrylate, and hydroxyethyl(meth)acrylate; 2) (meth)acrylamide and derivatives thereof such asN-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, andN,N-dimethyl(meth)acrylamide; 3) basic unsaturated monomers and salts orquaternized derivatives thereof, such as dimethylaminoethyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide, vinylpyridine, and2-vinylimidazole; 4) carboxyl-containing unsaturated monomers and saltsthereof, such as (meth) acrylic acid, itaconic acid, maleic acid, andfumaric acid; 5) anhydrides of unsaturated acids, such as maleicanhydride and itaconic anhydride; 6) vinyl esters such as vinyl acetateand vinyl propionate; 7) vinylethylene carbonate and derivativesthereof; 8) styrene and derivatives thereof; 9) 2-sulfonylethyl(meth)acrylate and derivatives thereof; 10) vinylsulfonic acids andderivatives thereof; 11) vinyl ethers such as methyl vinyl ether, ethylvinyl ether, and butyl vinyl ether; and 12) olefins such as ethylene,propylene, octene, and butadiene. Of those, the comonomers 1) to 7) areparticularly preferable from the standpoint of copolymerizability withN-vinyl compounds. The comonomers enumerated above may be used alone orin combination of two or more thereof in copolymerization.

The proportion of the N-vinyl compounds, such as vinylpyrrolidone andvinylcaprolactam, in all the monomer components constituting the N-vinylcompound polymer is not particularly limited. However, the proportionthereof is preferably 1.0 mol % or higher, more preferably 10.0 mol % orhigher, most preferably 20.0 mol % or higher, based on all the monomercomponents. Proportion of the N-vinyl compound in the copolymer lowerthan 0.1 mol % is undesirable in that various properties attributable toN-vinyl compounds cannot be imparted to the copolymer.

The method of the invention is effective as a method for diminishingresidual monomers in those N-vinyl compound polymers, and is preferablyapplied to N-vinyllactam polymers such as poly(N-vinylpyrrolidone), andis also preferably applied to N-vinyllactam polymers, such asN-vinylpyrrolidone, that are used in medicines or cosmetics, in whichdiminishing residual monomers is strongly demanded, and also to whichregulation and stabilization of pH and molecular weight are stronglydemanded in the fields of electronic materials and the like.

For obtaining an N-vinyl compound polymer, a polymerization reaction canbe conducted by the conventional methods, e.g., bulk polymerization,solution polymerization, emulsion polymerization, suspensionpolymerization or precipitation polymerization. The polymerizationreaction maybe conducted at suitable temperature according to conditionsincluding reactants. However, the reaction temperature is preferably 0to 250° C., more preferably 20 to 150° C., most preferably 40 to 100° C.

During the polymerization reaction, the reaction system may have anydesired internal pressure. However, ordinary or elevated pressure ispreferred when the reaction is conducted at high temperature. In thecase where strict temperature control is necessary, it is preferred toconduct the reaction at ordinary pressure.

In conducting a polymerization reaction for obtaining an N-vinylcompound polymer, conventional polymerization initiators can be added.Examples of the initiator include free-radical polymerization initiatorssuch as azo compounds, e.g., 2,2′-azobisisobutyronitrile and2,2′-azobis(2-methylpropionamidine) dihydrochloride, and peroxides suchas benzoyl peroxide and hydrogen peroxide; cationic polymerizationinitiators such as boron trifluoride and complexes thereof, iron (II)chloride, diethylaluminum chloride, diethylzinc, heteropolyacids, andactivated clay; persulfates such as potassium persulfate, ammoniumpersulfate, and sodium persulfate; and redox initiators which areoxidizing agent/reducing agent combinations generating free radicals,such as a combination of ascorbic acid and hydrogen peroxide,combination of sodium sulfoxylate and t-butyl hydroperoxide, andcombination of a persulfate and a metal salt. Although the concentrationof the polymerization initiator in conducting the polymerizationreaction is not particularly limited, it is preferably 0.001 to 10% byweight (hereinafter referred to as wt %), more preferably 0.005 to 5 wt%, most preferably 0.01 to 3 wt %, based on the weight of the monomercomponents. Besides the polymerization initiator, any desired additivesmay be suitably used according to need in conducting the polymerizationreaction, such as a chain transfer agent, pH regulator, and bufferingagent.

Solvents which can be used for the polymerization reaction for obtainingan N-vinyl compound polymer are not particularly limited. Examplesthereof include water; alcohols such as methyl alcohol, ethyl alcohol,isopropyl alcohol, and diethylene glycol; alkylene glycol ethers (etheracetates) such as propylene glycol monomethyl ether acetate anddiethylene glycol monomethyl ether acetate; amides such asdimethylformamide and N-methylpyrrolidone; esters such as ethyl acetate,butyl acetate, and γ-butyrolactone; aliphatic hydrocarbons such ashexane and octane; alicyclic saturated hydrocarbons such as cyclohexane;alicyclic unsaturated hydrocarbons such as cyclohexene; aromatichydrocarbons such as benzene, toluene, and xylene; ketones such asacetone and methyl ethyl ketone; halogenated hydrocarbons such asdichloroethane, chloroform, and carbon tetrachloride; ethers such asdiethyl ether, dioxane, and tetrahydrofuran; sulfonic esters such asdimethyl sulfoxide; carbonic esters such as dimethyl carbonate anddiethyl carbonate; and alicyclic carbonic esters such as ethylenecarbonate and propylene carbonate. Of these solvents, ethers (etheracetates) and amides are preferably used, and water and alcohols aremore preferably used. These solvents may be used alone or as a mixtureof two or more thereof. These solvents are preferably used in an amountsuch that the concentration of the monomer components in the reactionmixture during polymerization is preferably 1 to 99 wt %, morepreferably 5 to 70 wt %, most preferably 10 to 60 wt %.

The term “N-vinyl compound polymer” as used herein means one which mayfurther contain residual monomers.

In the invention, an organic acid having a boiling point of 140° C. orhigher at ordinary pressure is added to an aqueous solution of anN-vinyl compound polymer, such as a vinylpyrrolidone polymer, wherebythe residual monomer contained in the polymer is diminished.

In the case where an N-vinyl compound polymer to be used is onesynthesized with a non-aqueous solvent, the solvent is replaced withwater or an aqueous solvent before the treatment of the polymer.

In the aqueous solution of an N-vinyl compound polymer to be treated bythe invention, the term “aqueous” means that the solvent is water or awater/organic solvent mixture. The organic solvent to be mixed withwater is not particularly limited so long as it is compatible withwater. However, it is preferably an organic solvent comprising methanolor ethanol.

The organic acid to be used in the invention is not particularly limitedso long as it has a boiling point of 140° C. or higher, and organiccompounds having acid groups such as carboxyl group, sulfonic group,phosphoric group, sulfuric group and phosphoric group can be used.Examples of the organic compound include oxalic acid, succinic acid,aspartic acid, citric acid, glutamic acid, fumaric acid, malic acid,maleic acid, phthalic acid, trimellitic acid, pyromellitic acid,propionic acid, heptanoic acid, octanoic acid, glycolic acid, salicylicacid, lactic acid, L-ascorbic acid, benzoic acid, methanesulfonic acid,benzenesulfonic acid, laurylbeneznesulfonic acid, p-toluenesulfonicacid, benzenephosphric acid and lauryl sulfate. Of those, oxalic acid,succinic acid, aspertic acid, citric acid, glutamic acid, fumaric acid,malic acid, propionic acid, heptanioc acid, octanoic acid, glycolicacid, salicylic acid, lactic acid, L-ascorbic acid and benzoic acid arepreferably used from the standpoint of safety. Further, oxalic acid andsuccinic acid are more preferably used from the standpoints of reduceddiscoloration, ease of pH control, etc. It is, of course, possible touse a mixture of two or more acids. For example, an oxalic acid/succinicacid mixture, oxalic acid/L-ascorbic acid mixture, succinicacid/L-ascorbic acid mixture, succinic-acid/oxalic acid/L-ascorbic acidmixture, oxalic acid/propionic acid mixture, oxalic acid/heptanoic acidmixture, or succinic acid/propionic acid mixture can be used.

The amount of the organic acid added is not particularly limited.However, the amount thereof is preferably 0.0001 to 5 wt %, morepreferably 0.001 to 0.5 wt %, based on the weight of the aqueous N-vinylcompound polymer solution. If the addition amount thereof is smallerthan 0.0001 wt %, there are cases where the effect of residual monomerdiminution is insufficient. If the amount thereof exceeds 5 wt %, thereare cases where the polymer suffers a decrease in molecular weight ordiscoloration.

Although the viscosity of the aqueous N-vinyl compound polymer solutionto be treated in the invention is not particularly limited, it ispreferably 100,000 mPa•s or lower at room temperature. If the viscosityof the solution exceeds 100,000 mPa•s at room temperature, it isdifficult to stir the solution by ordinary methods and there are caseswhere the effect of residual monomer diminution is insufficient.

The pH of the aqueous N-vinyl compound polymer solution to which an acidhas been added is not particularly limited. However, the range of the pHthereof is preferably 1.5 to 6.0, more preferably 2.5 to 5.0. If the pHof the solution is lower than 1.5, there are cases where the N-vinylcompound polymer suffers a decrease in molecular weight. If the pHthereof exceeds 6.0, there are cases where the effect of residualmonomer diminution is insufficient.

The temperature at which an acid is added to treat the aqueous N-vinylcompound polymer solution is not particularly limited. However, thetemperature is preferably 25 to 150° C., more preferably 50 to 100° C.If the temperature is lower than 25° C., there are cases where theeffect of residual monomer diminution is insufficient. If thetemperature exceeds 150° C., there are cases where the polymer suffers adecrease in molecular weight or discoloration.

The time over which the polymer solution is maintained at thattemperature after the acid addition is not particularly limited.However, the time is preferably 5 minutes to 24 hours, more preferably10 minutes to 6 hours. If the time is shorter than 5 minutes, there arecases where the effect of residual monomer diminution is insufficient.If this period exceeds 24 hours, there are cases where the polymersuffers a decrease in molecular weight or discoloration.

Methods for adding an acid in the invention are not particularlylimited. The acids may be added alone or may be added as a solution inwater or an organic solvent.

On the other hand, after the addition of an acid, the resulting systemmay be allowed to stand. It is, however, preferred to stir the systemfrom the standpoint of effectively removing the residual monomer.Especially in the case where the aqueous polymer solution has aviscosity of 100 mPa.s or higher, it is preferred to stir the solutionwith a stirring impeller for high-viscosity fluids, e.g., Max BlendImpeller or Super Blend Impeller, both manufactured by Sumitomo HeavyIndustries Ltd.

In the process of the invention for producing an aqueous N-vinylcompound polymer solution which comprises adding an organic base to anaqueous polymer solution having a pH lower than 7.0 to thereby regulatethe pH thereof, the organic base is not particularly limited. Examplesthereof include monoethanolamine, diethanolamine, triethanolamine,guanidine carbonate, dihydrazide adipate, allylamine, diallylamine,triallylamine, isopropylamine, diisopropylamine, diaminopropylamine,ethylamine, diethylamine, triethylamine, 2-ethylhexylamine,3-(2-ethylhexyloxy)propylamine, 3-ethoxypropylamine, diisobutylamine,3-(diethylamino)propylamine, di-2-ethylhexylamine,3-(dibutylamino)propylamine, tetramethylethylenediamine,hexamethylenediamine, tri-n-octylamine, t-butylamine, sec-butylamine,propylamine, 3-(methylamino)propylamine, 3-(dimethylamino)propylamine,N-methyl-3,3′-iminobis(propylamine), and 3-methoxypropylamine. Of these,the organic bases having a boiling point of 120° C. or higher atordinary pressure are preferable, and triethanolamine, guanidinecarbonate and dihydrazide adipate are particularly preferably usedbecause pH regulation with these bases is easy and they have a highboiling point and, hence, cause no decrease in pH upon drying.

The term “pH” as used herein for an aqueous N-vinyl compound polymersolution used in the invention means the value of pH obtained bymeasuring the pH of the solution at 25° C. without changing the originalconcentration thereof.

The amount of the organic base to be added and the pH of the aqueousN-vinyl compound polymer solution after addition of the organic base arenot particularly limited. However, the pH of the aqueous N-vinylcompound polymer solution is regulated to preferably 6.0 or higher, morepreferably 7.0 or higher, most preferably 7.0 to 9.0. The amount of theorganic base to be added depends on the kinds of the acid and organicbase used. For example, the amount thereof is preferably 0.0001 to 5 wt%, more preferably 0.001 to 0.5 wt %, based on the weight of the aqueousN-vinyl compound polymer solution. If the pH of the solution to whichthe organic base has been added is lower than 6.0, there are cases wherethe N-vinyl compound polymer suffers a decrease in molecular weight withthe lapse of time or upon drying.

The temperature of the aqueous N-vinyl compound polymer solution duringaddition of an organic base thereto is not particularly limited.However, the temperature thereof is preferably 10 to 150° C., morepreferably 20 to 100° C. If the reaction temperature is lower than 10°C., there are cases where the neutralization reaction is insufficient.If the reaction temperature exceeds 150° C., there are cases where theN-vinyl compound polymer suffers a decrease in molecular weight ordiscoloration.

In the case where an acid is added to the aqueous N-vinyl compoundpolymer solution prior to the addition of an organic base thereto in theinvention, the time interval of from the acid addition to the additionof the organic base is not particularly limited. However, the timeinterval is preferably 5 minutes to 24 hours, more preferably 10 minutesto 6 hours. If the time interval is shorter than 5 minutes, there arecases where the effect of the acid addition is insufficient when theacid is intended to diminish a residual monomer. If the time intervalexceeds 24 hours, there are cases where the N-vinyl compound polymersuffers a decrease in molecular weight or discoloration before theaddition of the organic base.

Methods for the addition of an organic base in the invention are notparticularly limited. The organic base may be added alone or may beadded as a solution in water or an organic solvent.

On the other hand, after addition of the organic base, the resultingsystem may be allowed to stand. It is, however, preferred to stir thesystem from the standpoint of effectively conducting the neutralizationreaction. Especially in the case where the aqueous N-vinyl compoundpolymer solution has a viscosity of 100 mPa.s or higher, it is preferredto stir the solution with the above-described stirring impeller forhigh-viscosity fluids.

In the diminution of a residual N-vinyl compound monomer and/or the pHregulation of an aqueous N-vinyl compound polymer solution, particularlyin diminishing the residual monomers by adding an acid to an aqueousN-vinyl compound polymer solution, it is preferred to regulate thegaseous phase in the reactor so as to have an oxygen concentration ofpreferably 5 vol % or lower, more preferably 1 vol % or lower, mostpreferably 0.1 vol % or lower. This technique is effective in preventingthe N-vinyl compound polymer from suffering a decrease in molecularweight or discoloration. Even in the case of treatment under such a lowoxygen concentration, use of an organic acid having a boiling point of140° C. or higher as an acid added preferable because molecular weightdecrease prevention effect and coloration prevention effect can surelybe achieved.

The oxygen concentration can be easily measured with a commerciallyavailable oxygen analyzer such as a galvanic cell type diffusionanalyzer or a zirconia sensor type analyzer.

According to the methods of the present invention, the amount ofresidual monomers in the polymer can easily be diminished to less than100 ppm to less than 10 ppm. As a result, there is no fear of unpleasantodor and toxicity, and stable products having excellent quality with nochange in pH and molecular weight can be obtained.

It is effective to further incorporate an antioxidant or the like intothe N-vinyl compound polymer of the invention in order to enhance thelong-term stability of the polymer.

If required and necessary, the N-vinyl compound polymer produced by theinvention may further contain various additives such as processingstabilizers, plasticizers, dispersants, fillers, age resistors, pigmentsand hardeners.

The invention will be explained below in more detail by reference toSynthesis Examples and Examples according to the invention. However, theinvention should not be construed as being limited by these Examples inany way. In the Synthesis Examples and Examples, the value of K of avinyllactam polymer was determined by measuring the relative viscosityof a 1 wt % aqueous solution of the polymer with a capillary viscometerat 25° C. and calculating the K value from the viscosity using thefollowing Fikentscher's equations:(logη_(rel))/C=[(75K ₀ ²)/(1+1.5K ₀ C)]+K ₀K=1,000K₀wherein C indicates the number of grams of the vinyllactam polymerpresent in 100 ml of the solution, and η_(rel) indicates the viscosityof the solution relative to that of the solvent.

SYNTHESIS EXAMPLE 1

3,200 g of water was introduced into a 5 liters flask equipped with alarge-impeller stirrer, monomer feed vessel, thermometer, cooling pipeand nitrogen gas inlet pipe. Nitrogen gas was introduced into the flask,and the contents were heated with stirring so as to maintain theinternal temperature of the flask at 75° C. 800 g of N-vinylpyrrolidoneand 0.24 g of 2,2-azobis (2-amidino propane) dihydrochloride weresupplied to the flask over 30 minutes, and polymerization was conducted.After the reaction mixture was heated at that temperature for 2 hours,the internal temperature was elevated to 90° C. and the mixture washeated for further 30 minutes to complete the polymerization. Thus, anaqueous solution of polyvinylpyrrolidone was obtained. The amount of theunreacted N-vinylpyrrolidone remaining in the aqueouspolyvinylpyrrolidone solution obtained was 550 ppm of all theN-vinylpyrrolidone supplied. The value of K of the aqueouspolyvinylpyrrolidone solution obtained was 90.

SYNTHESIS EXAMPLE 2

2,700 g of water and 6 g of 10 ppm aqueous copper acetate solution wereintroduced into the same type of the flask as used in SynthesisExample 1. Nitrogen gas was introduced into the flask, and the contentswere heated with stirring so as to maintain the internal temperature ofthe flask at 100° C. 30 g of 2% ammonia water, 210 g ofN-vinylpyrrolidone, and 10 g of 4% aqueous hydrogen peroxide solutionwere supplied to the flask three times at intervals of 5 minutes toconduct polymerization. The reaction mixture was stirred at thattemperature for 2 hours to complete the polymerization. Thus, an aqueoussolution of polyvinylpyrrolidone was obtained. The amount of theunreacted N-vinylpyrrolidone remaining in the aqueouspolyvinylpyrrolidone solution obtained was 480 ppm. The value of K ofthe aqueous polyvinylpyrrolidone solution was 31.

EXAMPLE 1

400 g of the aqueous polyvinylpyrrolidone solution obtained in SynthesisExample 1 was introduced into a 500 ml reactor equipped with the sameattachment devices as in Synthesis Example 1. Nitrogen gas wasintroduced into the reactor through the nitrogen gas inlet pipe so thatthe gaseous phase in the reactor had an oxygen concentration of 1 vol %.Thereafter, the contents were heated to 75° C. After completion of theheating, an aqueous solution prepared by dissolving 0.096 g of oxalicacid in 9.5 g of water was added to the contents and the resultingmixture was stirred for 60 minutes. The aqueous polyvinylpyrrolidonesolution thus obtained had a pH of 3.2, and the amount of the unreactedN-vinylpyrrolidone remaining there in was 2 ppm. The value of K of thethus-obtained aqueous polyvinylpyrrolidone solution just after thereaction was 89. When this aqueous solution was stored in air at 50° C.for 2 weeks, the value of K thereof decreased to 85.

EXAMPLE 2

400 g of the aqueous polyvinylpyrrolidone solution obtained in SynthesisExample 1 was introduced into the same type of the reactor as used inExample 1. The contents were heated to 95° C. while regulating thegaseous phase in the reaction vessel so as to have an oxygenconcentration of 0.1 vol %. After completion of the heating, an aqueoussolution prepared by dissolving 0.096 g of succinic acid in 9.5 g ofwater was added to the contents and the resulting mixture was stirredfor 60 minutes. The aqueous polyvinylpyrrolidone solution thus obtainedhad a pH of 4.0, and the amount of the unreacted N-vinylpyrrolidoneremaining therein was 7 ppm. The value of K of the thus-obtained aqueouspolyvinylpyrrolidone solution just after the reaction was 90. When thisaqueous solution was stored in air at 50° C. for 2 weeks, the value of Kthereof decreased to 86.

COMPARATIVE EXAMPLE 1

The same procedure as in Example 1 was followed, except that theintroduction of nitrogen gas was omitted. During the treatment, thegaseous phase in the reaction vessel had an oxygen concentration of 10.0vol %. The aqueous polyvinylpyrrolidone solution thus obtained had a pHof 3.2, and the amount of the unreacted N-vinylpyrrolidone remainingtherein was 2 ppm. The value of K of the thus-obtained aqueouspolyvinylpyrrolidone solution just after the reaction was 86. When thisaqueous solution was stored in air at 50° C. for 2 weeks, the value of Kthereof decreased to 78.

EXAMPLE 3

The aqueous polyvinylpyrrolidone solution obtained in Synthesis Example2 was introduced into the same type of the reactor as used in SynthesisExample 2. While the contents were maintained at 75° C., an aqueoussolution prepared by dissolving 0.096 g of oxalic acid in 9.504 g ofwater was added thereto. The resulting mixture was stirred for 60minutes while introducing nitrogen gas through the nitrogen gas inletpipe so that the gaseous phase in the reaction vessel had an oxygenconcentration of 1 vol %. The aqueous polyvinylpyrrolidone solution thusobtained had a pH of 3.1, and the amount of the unreactedN-vinylpyrrolidone remaining therein was 0.7 ppm. The value of K of thethus-obtained aqueous polyvinylpyrrolidone solution just after thereaction was 31.

EXAMPLE 4

The aqueous polyvinylpyrrolidone solution obtained in Synthesis Example2 was introduced into the same type of the reactor as used in SynthesisExample 2. The contents were heated to 95° C. After completion of theheating, an aqueous solution prepared by dissolving 0.096 g of succinicacid in 9.504 g of water was added thereto. The resulting mixture wasstirred for 60 minutes while introducing nitrogen gas through thenitrogen gas inlet pipe so that the gaseous phase in the reaction vesselhad an oxygen concentration of 1 vol %. The aqueous polyvinylpyrrolidonesolution thus obtained had a pH of 4.0, and the amount of the unreactedN-vinylpyrrolidone remaining the rein was 2 ppm. The value of K of thethus-obtained aqueous polyvinylpyrrolidone solution just after thereaction was 31.

EXAMPLE 5

An aqueous solution prepared by dissolving 0.35 g of triethanolamine in17.15 g of water was added to the aqueous polyvinylpyrrolidone solutionobtained in Example 1 just after the reaction. The resulting mixture wasstirred for 30 minutes. The aqueous polyvinylpyrrolidone solution thusobtained had a pH of 7.2, and the amount of the unreactedN-vinylpyrrolidone remaining there in was 2 ppm. The value of K of thethus-obtained aqueous polyvinylpyrrolidone solution just after thereaction was 89. When this aqueous solution was stored in air at 50° C.for 2 weeks, the value of K thereof remained at 89.

EXAMPLE 6

An aqueous solution prepared by dissolving 0.36 g of guanidine carbonatein 17.64 g of water was added to the aqueous polyvinylpyrrolidonesolution obtained in Example 2 just after the reaction. The resultingmixture was stirred for 30 minutes. The aqueous polyvinylpyrrolidonesolution thus obtained had a pH of 8.1, and the amount of the unreactedN-vinylpyrrolidone remaining therein was 7 ppm. The value of K of thethus-obtained aqueous polyvinylpyrrolidone solution just after thereaction was 90. When this aqueous solution was stored in air at 50° C.for 2 weeks, the value of K thereof remained at 90.

EXAMPLE 7

The aqueous polyvinylpyrrolidone solution obtained in Example 5 was caston a Teflon sheet and then dried in a hot-air drying oven first at 105°C. for 2 hours and subsequently at 150° C. for 10 minutes. The solidthus obtained was pulverized to obtain a polyvinylpyrrolidone powder.The value of K of the polyvinylpyrrolidone obtained was 90, and theamount of the unreacted N-vinylpyrrolidone remaining therein was 2 ppm.A 10 wt % aqueous solution of the polyvinylpyrrolidone powder obtainedhad a pH of 7.2.

EXAMPLE 8

The aqueous polyvinylpyrrolidone solution obtained in Example 6 wassubjected to the same treatment as in Example 7 to obtain apolyvinylpyrrolidone powder. The value of K of the polyvinylpyrrolidoneobtained was 90, and the amount of the unreacted N-vinylpyrrolidoneremaining therein was 6 ppm. A 10 wt % aqueous solution of thepolyvinylpyrrolidone powder obtained had a pH of 8.1.

COMPARATIVE EXAMPLE 2

The aqueous polyvinylpyrrolidone solution obtained in Example 1 was,without pH adjustment, cast on a Teflon sheet and then dried in ahot-air drying oven first at 105° C. for 2 hours and subsequently at150° C. for 10 minutes. The solid thus obtained was pulverized to obtaina polyvinylpyrrolidone powder. The amount of the unreactedN-vinylpyrrolidone remaining in the polyvinylpyrrolidone obtained was 2ppm. A 10 wt % aqueous solution of the polyvinylpyrrolidone powderobtained had a pH of 3.1, and the value of K of this polymer haddecreased to 82 as compared with 90 in Example 7.

1. A process for producing an N-vinyl compound polymer which comprisescontaining an organic base in an N-vinyl compound polymer to regulate pHof the polymer, wherein the organic base comprises at least one memberselected from the group consisting of triethanolamine, guanidinecarbonate and dihydrazide adipate.
 2. A process for producing an aqueousN-vinyl compound polymer solution which-comprises adding an organic baseto an aqueous N-vinyl compound polymer solution having a pH lower than7.0 to thereby regulate pH of the solution, wherein the organic basecomprises at least one member selected from the group consisting oftriethanolamine, guanidine carbonate and dihydrazide adipate.
 3. Aprocess for producing an N-vinyl compound polymer powder which comprisesadding an organic base to an aqueous N-vinyl compound polymer solutionhaving a pH lower than 7.0 to thereby regulate pH of the solution anddrying the resulting aqueous polymer solution, wherein the organic basecomprises at least one member selected from the group consisting oftriethanolamine, guanidine carbonate and dihydrazide adipate.
 4. Theprocess of claim 1, 2 or 3, wherein said organic base is an organic basehaving a boiling point of 120° C. or higher at ordinary pressure.
 5. Theprocess as claimed in claim 1, 2 or 3, wherein said N-vinyl compound isN-vinyllactams.
 6. The process as claimed in claim 1, 2 or 3, whereinsaid N-vinyl compound is N-vinylpyrrolidone.